In conventional hydraulic systems, pressurized hydraulic fluid is supplied from a pump to a cylinder (actuator) and hydraulic fluid flows out of the actuator to a tank. The flow to the actuator and out of the actuator is controlled by a spool valve. The position of a spool within the spool valve controls the flow of the hydraulic fluid. When the spool valve is actuated, the spool moves to a certain position and controls the flow of hydraulic fluid both to and from the actuator.
The construction of the four way spool valve is such that a given position of the spool determines the ‘flow in’ and the ‘flow out’ restriction sizes. Thus, metering-in and metering-out are coupled, and a certain restriction size on the inlet corresponds to a certain restriction size on the outlet. Therefore, the valve has one degree of freedom, and can control either the speed of the actuator or the pressure in one chamber of the actuator but not both. Thus, a spool valve can provide for speed control but it cannot achieve energy saving potential at the same time.
Further, in the case of an overrunning load, which occurs when lowering a load with gravity assistance, for example, a spool valve is designed such that the outlet restriction is used to control the flow so as to prevent the load from falling at uncontrollable speeds. However, in other operating conditions, such as lifting a load, this restriction is not needed yet it is inherent in the design of the spool valve and causes energy loss.
Further, some spool valves are actuated via a pilot fluid signal. Particularly, a pressure reducing valve is added to the system and is configured to receive fluid from the pump and reduce the pressure level of the fluid before providing the pilot signal fluid having the reduced pressure level to the spool valve to move its spool. Having such a pressure reducing valve can increase the cost of the hydraulic system. Further, such a hydraulic system involves first reducing pressure level then controlling a stroke of the spool of the spool valve. Such configuration can lead to poor resolution and poor proportionality between a command signal to the pressure reducing valve and a position of the spool.
It may thus be desirable to have a hydraulic system that replaces the spool valve with two independently-controlled meter-in valves. It may also be desirable to have meter-in valves with a mechanical feedback to control stroke of a movable element rather than controlling pressure level. This way, the pressure reducing valve is eliminated from the system and enhanced flow resolution and proportionality can be achieved. It is with respect to these and other considerations that the disclosure made herein is presented.